Others will have specific advice for you on this, but why do you think you need tires that can carry heavier loads. Being fairly beefy tires (at least by modern standards), I would think those tires could lift quite a load off the ground. Are you airing them up as much as you can? After all, its air, not rubber that keeps your rim off the road.

You can feel free to also ask in the Clydesdale section, but I weigh 350 pounds and I ride any 27" tire.

A couple of my favorites are the same that others here like. The Panaracer Pasela TG or the Continental Ultra Sport both ride well.

Just be sure to keep the tires well inflated to avoid pinch flats. If your rims don't have hooked beads (and I don't think they do if they are the stock rims) then stick to under 80 PSI or so on the inflation, regardless of what the tire sidewalls say.

If you are staying at 80 PSI and having trouble with pinch flats, I would suggest switching to hooked rims to allow for higher pressure.